Multicarrier CDMA (hereinafter called “MC-CDMA”) that combines a multicarrier transmission technology with a spread spectrum has conventionally gained attention as a major candidate for a next-generation mobile communication technology. In this MC-CDMA, spectrum spreading is performed on a data symbol and signals after spreading are allocated to subcarriers of OFDM. FIG. 15 is a block diagram showing an outline configuration of a transmitter that performs an MC-CDMA communication. In a transmitter 150, data to be transmitted is added with an error correcting code by a coding part 151 and modulated by a modulation part 152.
A multiplexing part (Mux part) 153 multiplexes the modulated data parallel-converted into as many portions as the number of subcarriers with a pilot signal used by a receiving side for estimating received SIR. Then, an S/P conversion part 154 converts the data into parallel signals. Further, each signal is copied by a copying part 155 to generate a plurality of signals. Here, the number of copied signals is equal to a spreading factor. That is, if the spreading factor is 16, for example, a signal is copied to generate 16 signals. Each copied signal is multiplied in order by a spread code generated by a spread code generation part 156 in a multiplier 157.
Thereafter, Fourier transform processing is performed by an IFFT part 158 and guard intervals are inserted by a guard interval insertion part 159 to generate an OFDM signal. Here, each subcarrier will be spread in order. That is, for SB1 (subcarrier 1) to SB16, the first data symbol is spread by spread codes 1 to 16, and for SB17 to SB32, the second data symbol is spread by the spread codes 1 to 16. Since original data symbols are converted into 16-fold spread symbols by spread processing, as described above, though the transmission rate drops to 1/16, multiplexing can be performed using different spread codes for a spread spectrum and therefore, the transmission rate can still be maintained.
FIG. 16 is a diagram showing an aspect of a spread symbol and a despread symbol. As shown in FIG. 16, each spread symbol is transmitted in a multiplexed form, but when despread, codes are orthogonal to one another and multiplexed signals do not interfere with one another and thus can completely be demultiplexed. That is, the following equation holds between spread codes a(t) and b(t):
                                          ∑                          t              =              1                        16                    ⁢                                    a              ⁡                              (                t                )                                      *                          b              ⁡                              (                t                )                                                    ≠        0                            [                  Eq          .                                          ⁢          1                ]            
Since up to 16 signals that are completely orthogonal to one another can be taken for the spreading factor of 16, as shown above, the transmission rate when multiplexed 16-fold will be the same as when no spreading occurs, eliminating completely an influence of rate deterioration due to a spread spectrum.
As a modification of MC-CDMA, on the other hand, a method of spreading using two dimensions of the frequency and time axes has been proposed. In this two-dimensional spreading, one data modulated symbol is spread over SFTime continuous OFCDM (Orthogonal Frequency and Code Division Multiplexing) symbols and SFFreq continuous subcarriers, and the overall spreading factor can be represented by SF=SFTime×SFFreq. Here, SFTime represents the spreading factor in a time dimension and SFFreq represents the spreading factor in a frequency dimension.
In the two-dimensional spreading, the overall spreading factor is controlled in accordance with a cell configuration. That is, a mobile station sets the spreading factor based on control information from a base station. Further, by adaptively controlling SFTime and SFFreq in accordance with propagation conditions, channel loads, radio parameters and the like, channel capacities are attempted to increase in both cellular systems and isolated cell environments.    [Non-Patent Document 1]: Shingaku Gihou RCS2000-136 “Study on Broadband Packet Wireless Access”    [Non-Patent Document 2]: NTT DoCoMo Technical Journal Vol. 5, No. 2 “Feature Story of 4th Generation Wireless Access Technology”